Glypican 1 Stimulates S Phase Entry and DNA Replication in Human Glioma Cells and Normal Astrocytes

Abstract

Malignant gliomas are highly lethal neoplasms with limited treatment options. We previously found that the heparan sulfate proteoglycan glypican 1 (GPC1) is universally and highly expressed in human gliomas. In this study, we investigated the biological activity of GPC1 expression in both human glioma cells and normal astrocytes in vitro. Expression of GPC1 inactivates the G₁/S checkpoint and strongly stimulates DNA replication. Constitutive expression of GPC1 causes DNA rereplication and DNA damage, suggesting a mutagenic activity for GPC1. GPC1 expression leads to a significant downregulation of the tumor suppressors pRb, Cip/Kip cyclin-dependent kinase inhibitors (CKIs), and CDH1, and upregulation of the pro-oncogenic proteins cyclin E, cyclin-dependent kinase 2 (CDK2), Skp2, and Cdt1. These GPC1-induced changes are accompanied by a significant reduction in all types of D cyclins, which is independent of serum supplementation. It is likely that GPC1 stimulates the so-called Skp2 autoinduction loop, independent of cyclin D-CDK4/6. Knockdown of Skp2, CDK2, or cyclin E, three key elements within the network modulated by GPC1, results in a reduction of the S phase and aneuploid fractions, implying a functional role for these regulators in GPC1-induced S phase entry and DNA rereplication. In addition, a significant activation of both the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathways by GPC1 is seen in normal human astrocytes even in the presence of growth factor supplement. Both pathways are constitutively activated in human gliomas. The surprising magnitude and the mitogenic and mutagenic nature of the effect exerted by GPC1 on the cell cycle imply that GPC1 may play an important role in both glioma tumorigenesis and growth.

View retraction statement:

Retraction for Qiao et al., Glypican 1 Stimulates S Phase Entry and DNA Replication in Human Glioma Cells and Normal Astrocytes

ACKNOWLEDGMENTS

This work was supported by UWCCC core grant P30 CA014520 and by award number I01BX000137 from the Biomedical Laboratory Research and Development Service of the VA Office of Research and Development.

We acknowledge the UWCCC Flow Cytometry Laboratory, a Shared Service of the UW Carbone Cancer Center, Madison, WI. We thank J. Lees (MIT Center for Cancer Research, Cambridge, MA) and Geoffrey M. Wahl (Gene Expression Laboratory, Salk Institute for Biological Studies, La Jolla, CA) for providing DNA constructs. We thank John Kuo (Department of Neurological Surgery, University of Wisconsin, Madison, WI) for providing human NSCs and GSCs and Rob Lera in Mark Burkard's laboratory (Department of Medicine, University of Wisconsin, Madison, WI) for his expert help with time-lapse live-cell imaging.

The contents of this paper do not represent the views of the Department of Veterans Affairs or the United States government.